Chlorine dioxide (ClO.sub.2) is a water-soluble gas, one of but a few compounds in nature which exist in a stable monomeric free radical form. In large part because of this structure, ClO.sub.2 is a highly efficient antimicrobial and antiviral agent, as well as an oxidant of noxious odor molecules. It has found increasing utility as a disinfectant, deodorant and bleaching agent, in areas ranging from municipal water disinfection, reduction of oral halitosis and decolorizing paper pulp. ClO.sub.2 is finding increased value as a replacement for chlorine in water and food disinfection because, unlike chlorine, it does not ordinarily produce chlorinated organic compounds which can be mutagenic and even carcinogenic. Unlike chlorine, it cannot be stored as a compressed gas, because the free radical molecule becomes explosive. As a result, for large scale use of ClO.sub.2 such as in water disinfection or in chiller tanks of poultry processors, the gas must be generated on site. This is accomplished either through reduction of a chlorate salt, or the oxidation of a chlorite. Acidification of chlorite also yields high quantities of ClO.sub.2 through disproportionation of the chlorous acid intermediate (Canadian Patent No. 959,238).
Aqueous ClO.sub.2 solutions are generally considered to be inherently unstable, and to-date no one has attempted to offer stable, free-molecular ClO.sub.2 solutions commercially, for its manifold applications. If a means were available to provide a simple aqueous solution of ClO.sub.2 for industrial and home use, it would provide the consumer with an efficient and safe means for disinfecting and deodorizing home and commercial areas in unparalleled ways.
Aqueous ClO.sub.2 solutions degrade in the following manner: 2 ClO.sub.2 +H.sub.2 O.revreaction.ClO.sub.2.sup.-- +ClO.sub.3 +2H.sup.+. Although acidic solutions suppress the degradation, it is largely complete even in fairly acid environments. A product, ProFresh.TM., which is an oral deodorizing solution containing free molecular ClO.sub.2, has been commercially distributed for a number of years, but its effective shelf life is limited to weeks or months because of continuing ClO.sub.2 loss. There is also a great deal of confusion relating to so-called "stabilized chlorine dioxide" solutions, which have little or none of the free ClO.sub.2 molecule, but which predominate instead in chlorite ion. The claim is made that during use, the unstable chlorite can lead to a slow generation of ClO.sub.2 but not with sufficient rapidity to provide any significant ClO.sub.2 activity. The "stabilization" of chlorine dioxide, by reaction of the ClO.sub.2 with peroxides to form chlorite, has been taught in a number of patents, including those of Wentworth (U.S. Pat. No. 3,123,521) and McNicholas (U.S. Pat. No. 3,271,242). Other attempts to stably contain ClO.sub.2 are found in U.S. Pat. No. 4,829,129, in which the molecule is claimed to be complexed with an organic polymer, and in U.S. Pat. No. 4,861,514, where ClO.sub.2 is apparently maintained in a steady-state concentration, after its slow formation over many days, in a thickened aqueous solution comprising a gelling agent, a chlorite salt, and an aldehyde or acetal. In neither of these two patents does the resulting composition provide a simple stable solution, of freely-available ClO.sub.2, appropriate for easy disinfecting or deodorizing applications, without the presence of other solutes necessary for ClO.sub.2 stabilization. In addition, the application of the referenced compositions to a substrate intended for disinfection, would leave significant levels of dried residue upon evaporation of the aqueous solvent.
U.S. Pat. No. 5,165,910 teaches a method for preparing what is termed by the inventor to be a stabilized aqueous chlorine dioxide solution, by reacting an excess of alkali metal chlorite with an alkali metal hypochlorite and an acid in solutions at a pH of either 7-9 or 2-5.6. The ClO.sub.2, is prepared from a solution comprising 0.1-5% of an alkali metal chlorite and 5-40 ppm of an alkali metal hypochlorite, which oxidizes a very small fraction of the metal chlorite to ClO.sub.2. The initial molar ratio of alkali metal chlorite to alkali hypochlorite is from about 20 to 6000, and the ratio of metal chlorite to ClO.sub.2 in the resulting solution is approximately the same. In the reference patent, the room-temperature storage of the resulting ClO.sub.2 solution for a month, in an unspecified container, indicated no appreciable loss of ClO.sub.2 at the alkaline pH, and only small losses at acidic pH's. When attempting to replicate these conditions, with storage of the resulting solutions in high-density polyethylene containers, the present inventor found that at least 50% of the ClO.sub.2 was lost within 14 days. Thus the patent is not instructive in terms of practicing the present invention and verifying the claimed stability of the resulting ClO.sub.2 solutions.
Accordingly, there is a long-felt need in the art for a means of providing a storage-stable chlorine dioxide solution, at concentrations appropriate for intermittent and extended use of the solution for a wide range of applications for which ClO.sub.2 is unique. These uses are in the home, industrial and agricultural settings, and include, among others, the disinfection of food items, kitchen countertops and bathroom fixtures; reduction of oral malodor by oxidation of the sulfurbearing compounds responsible for halitosis as well as destruction of the putrefactive microorganisms responsible for the production of noxious odorants; the deodorization of pet animals, including the odor of skunks; disinfection of fresh picked raw fruits, vegetables and related agricultural products; and the bleaching of stained clothing in a safe manner.
There is a more particular need to identify specific container materials, including certain preferred techniques for producing such container materials, so that ClO.sub.2 solutions may be properly maintained, with little or no loss of the ClO.sub.2 by reaction with the container and/or permeation therethrough. The present invention is the direct result of investigations into means for minimizing or preventing loss of ClO.sub.2 associated with the distribution of ClO.sub.2 -containing oral malodor treatment solutions in plastic containers, and led to surprising findings with respect to the properties of aqueous ClO.sub.2 solutions.